Method for producing nanoparticles from a noble metal and use of the nanoparticles thus produced

ABSTRACT

The invention relates to a method for producing nanoparticles from a noble metal and to the use of the nanoparticles produced using the method. It is the object of the invention to provide possibilities for a simple and inexpensive production of nanoparticles from a noble metal in which influence can be directly made on the particle size of the nanoparticles thus produced. Nanoparticles of silver, gold and platinum can be produced with an adjustable particle size using the method in accordance with the invention. A chemical compound of the respective noble metal is dissolved in an aqueous solution or the noble metal is dissolved in an acid mixture. An aqueous solution containing at least one surfactant is added to the respective solution and a reductant is additionally added with silver and platinum. The parameters: concentration of the chemical compound or of the noble metal, the temperature, pH and the fraction of surfactant are influenced to influence the particle size. The noble metal particles precipitated from the respective solution can be centrifuged out.

The invention relates to a method for producing nanoparticles from a noble metal and to the use of the nanoparticles produced using the method for the production of printable suspensions, inks or pastes for printing or for forming functional layers (e.g. electrically conductive layers) or decorative (e.g. optically reflective layers) surfaces. Nanoparticles of silver, gold or platinum should be produced using the method. The catalytic effect of these noble metals can also be utilized.

Nanoparticles of noble metals, in particular silver, are used for producing inks which can be applied to substrates using various application methods. Since the sintering behavior of such inks, and in this respect in particular the temperature required for a sufficient sintering, is influenced by the particle size and also by the particle size distribution, it is of great interest to be able to produce such nanoparticles from noble metal with a predefinable particle size and, optionally, also particle size distribution.

There are also chemical synthesis processes in addition to physical processes. It is thus known in principle from Yu-Chieh Lu et al.; from “A simple and effective route for the synthesis of nano-silver colloidal dispersions”; Journal of the Chinese Institute of Chemical Engineers; 39 (2008) p. 673-678, to produce nanoparticles from silver. In this respect, silver nitrate in an aqueous solution in which polyvinylpyrrolidine is contained should be reduced to silver nanoparticles using dextrose as a reductant. The basic synthesis is described therein. There are, however, no indications on how a direct influencing of the particle size can be achieved in this process. In addition, there are problems with completely removing the dextrose residue and the sodium residue from the synthesis without being able to avoid disadvantages in the production of the nanoparticles.

It is therefore the object of the invention to provide possibilities for a simple and inexpensive production of nanoparticles from a noble metal in which influence can be directly made on the particle size of the nanoparticles thus produced.

In accordance with the invention, this object is achieved by a method utilizing the features of claim 1. Advantageous further developments of the invention are shown by features contained in dependent claims. An advantageous use is set forth in claim 9.

In the method in accordance with the invention, a chemical compound of the respective noble metal should be dissolved in an aqueous solution or the coarse-grain noble metal should be dissolved in an acid mixture. At least one surfactant or an aqueous or alcohol solution containing at least one surfactant should be added to the respective solution; with silver and platinum a reductant should additionally be added.

The influencing of the particle size of the nanoparticles produced using the method takes place with the parameters: concentration of the chemical compound or of the noble metal; temperature; and the fraction of surfactant. The pH or the setting of specific pH values in individual method steps can also have an influence on the synthesis.

After the actual synthesis, the noble metal particles precipitated from the respective solution are centrifuged out.

The particular size can be reduced with a smaller concentration of the chemical compound used or of the noble metal in the respective solution and/or at an increased temperature and/or an increased fraction of surfactant.

An increased pH can in particular result in smaller particle sizes in the production of nanoparticles from silver or platinum.

These noble metals can be dissolved in a mixture of hydrochloric acid and nitric acid for the production of nanoparticles from gold or platinum. The mixture ratio should in this respect be 75% by weight hydrochloric acid, 25% by weight nitric acid. This acid mixture is also known as aqua regia. Tetrachloroauric acid or hexachloroplatinic acid then forms on the dissolving.

Pure platinum in the form of nanoparticles can precipitate by reduction by an addition of hydrazine hydrate.

With gold, this can also be achieved by a direct influencing of the pH without reductant.

There is also the possibility with silver of directly dissolving silver in an acid. Nitric acid of at least 50% can be used for this. The nitric acid should be heated and should have a temperature in the range 100° C. to 150° C., preferably 120° C. In this respect, AgNO₃ is formed and a surfactant and additionally hydroxylamine can again be added, preferably in aqueous solution, as a reductant.

The method is simplified by the use of the pure noble metals gold, silver and platinum which are dissolved directly in acid, and the costs can also be reduced since the use of suitable chemical compounds of these noble metals whose purchasing costs are higher than those of the pure noble metals can be dispensed with. In addition, smaller losses of noble metal occur since the fractions not converted into nanoparticles can be used again.

With all three of the noble metals looked at here, the use of the surfactant(s) has the result that an agglomeration of the individual nanoparticles can be avoided.

In the method in accordance with the invention, surfactants can be used which are selected from alkoxylates, alkylolamides, esters, amine oxides, alkyl polyglucosides, alkylphenols, arylalkyl phenols, water-soluble homopolymers, water-soluble statistical copolymers, water-soluble block copolymers, water soluble graft copolymers, polyvinyl alcohols, copolymers of polyvinyl alcohols and polyvinyl acetates, polyvinylpyrrolidones, cellulose, starch, gelatins, gelatin derivatives, amino acid polymers, polylysine, polyasparagine acid, polyacrylates, polyethylene sulfonates, polystyrene sulfonates, polymethacrylates, condensation products of aromatic sulfonic acids with formaldehyde, napththalenesulfonates, lignosulfonates, copolymerizates of acrylic monomers, polyethyleneimines, polyvinylamines, polyallylamines, poly(2-vinylpyridines), block copolyethers, block copolyethers with polystyrene blocks, sodium dodecylbenzenesulfonate, and polydiallyl dimethyl ammonium chloride.

In this respect, polyvinylpyrrolidones, block copolyethers and block copolyethers with polystyrene blocks, hydroxyl functional carboxylic acid esters with pigment-affinic groups, copolymers with pigment-affinic groups, preferably acidic groups, alkylolammonium salts of a block copolymer with pigment-affinic groups, preferably acidic groups, and/or mixtures or solutions hereof are particularly preferred.

Block copolymers with pigment-affinic groups such as polystyrene blocks (e.g. Disperbyk 190 of the company BYK-Chemie, Wesel), alkylolammonium salts of a copolymer with acidic groups (e.g. Disperbyk 180 of the company BYK-Chemie, Wesel) or polyvinylpyrrolidones (e.g. PVP of the company Fluka) or mixtures thereof can particularly preferably be used as surfactants. Dysperbyk 180 is an alkylammonium salt of a copolymer with acidic groups. Dysperbyk 190 is an aqueous solution of a high-molecular block copolymer with pigment-affinic groups.

Hydroxylamine is advantageous for the production of nanoparticles from silver and hydrazine hydrate or sodium borohydride can advantageously be used as a reductant for the production of nanoparticles from platinum.

The following chemical reactions can take place in the synthesis of silver nanoparticles with hydroxylamine as a reductant:

Ag+2HNO₃→AgNO₃+H₂O+NO₂

12NH₂OH+2AgNO₃→7N₂+15H₂O+2Ag+6H⁺

3NH₂OH→N₂+NH₃+3H₂O

The pH in the respective solution can be set during the synthesis using added NaOH or added NH₃. A setting of the pH is also possible using piperidine and thus free of sodium.

The production and use of nanoparticles produced in accordance with the invention will be described with reference to examples in the following.

EXAMPLE 1

32 g AgNO₃ (of the type AppliChem, purest) were dissolved in 1600 ml water and 22.1 g dispersant Disperbyk 180 and 5.1 g Disperbyk 190 (both from Byk-Chemie) were added. The solution was heated to 60° C. and set to a pH of 10 with the aid of NH₃. Subsequently, 20 ml hydroxylamine 50% aqueous solution (type Merck, for synthesis) was added. A volume of at least 5 liters must be provided for the described reaction due to the strong gas development in the reaction. The reaction comes to a stop after approx. 30 s. In this time, the temperature in the solution increases to around 65° C. The reaction vessel is cooled to room temperature while stirring continuously and is subsequently centrifuged. A separation of coarse particles larger than 300 nm at an acceleration of 800 g for 10 min. is suitable for the described reaction conditions. Subsequently, centrifuging takes place for 2 hours at a maximum acceleration of the centrifuge, e.g. at 4600 g. A yield of 75-80% of the silver mass used in nanoparticles is thus possible. The coarse particles can furthermore be burnt out of the adhering organic dispersant and can be returned to AgNO₃ again by subsequent conversion using boiling HNO₃.

A variation of the particle size of the nanoparticles to be obtained is possible by a variation of the concentration of the raw materials used. Table 1 provides an overview of 4 different synthesis approaches using the obtained mean particle sizes and the particle morphology in FESEM. REM images of nanoparticles in accordance with examples 1.1 and 1.2 from Table 1 are shown in FIG. 1.

The particles produced from the synthesis in accordance with Example 1 are further processed to a silver particle ink. The centrifuge sediment after 2 h of centrifuging at 4600 g is charged with as little water as possible for this purpose and is provided in a ball mill with 10% by weight PEG (polyethylene glycols) and 0.05% by weight Disperbyk 348 (figures with respect to the mass of the sediment) and dispersed over a time period of 0.5 hours. To ensure the avoidance of later clogging of printing nozzles, the ink thus obtained was liberated from any coarse agglomerates with the aid of a 5 μm steel filter. The viscosity of the ink amounts to 21 mPas at a shear rate of 100/s and at 25° C. in the cylinder beaker system (TA Instruments, DA100). The solid content of the ink is calculated using a density measurement and amounted to 54% by weight at a density of 2.05 g/cm³. An FESEM shot on polished Al is shown in FIG. 2.

This ink was deposited on silicon with the aid of a Dimatix SQ128 printhead. The layer thickness in a single print amounted to 2.5 μm, with a line width of 37 μm-40 μm. In multiprint with 10 repetitions, layers thicknesses between 18 μm and 20 μm are achieved, with line widths of 60 μm-65 μm. The printed structures were burnt in a tube furnace at a heating rate of 10 K/min up to a temperature of 380° C. The electrical resistance of the printed structure is simultaneously determined via a 4-point measurement in this process. The printed layer was previously dried at 180° C. for 0.5 h. This already allows it to become electrically conductive.

EXAMPLE 2

In a first step, 10 g of a commercial gold powder (Heraeus 200-03) were completely dissolved in at least 20 ml 75% by weight HCl with 25% by weight HNO3 at 50° C. Since contaminants are to be avoided, no metallic devices should be used for this purpose.

After cooling the solution, the solution was passed through a hard paper filter. 800 ml deionized water was mixed with the surfactants Disperbyk 180 and Disperbyk 190 using a magnetic stirrer in a glass beaker. The quantity of added surfactant in each case corresponded to 60% by weight of the previously converted gold mass. The acid solution was subsequently added into the water-surfactant mixture and was titrated to a neutral pH via the addition of NaOH (3 . . . 5 mol). A pH electrode was used for this purpose. The pH was first stabilized at pH 7 and then increased step-wise to a pH of 10 after a further 5 to 7 minutes. Gold nanoparticles were precipitated in the solution by this increase in the pH with a stabilization at pH 10.

Coarser particles were separated by the use of a centrifuge with 2000 r.p.m. (800 times the force of gravity g) for 10 min. and can thus be recycled.

A separation of finer particles with a mean particle size d₅₀ of <80 nm should be centrifuged again. In this respect, centrifuging should be carried out at a speed of 4700 r.p.m. (4600 times the force of gravity g) over a time period of 2 hours. The remaining gold particle sludge was washed with water to reduce the sodium content in the solution. In addition to gold, residues of sodium chloride were present from the synthesis which can be reduced by further washing of the solution containing particles. Particles having a size which were suitable for the production of printable ink jet ink were thus able to be obtained.

To produce a printable ink from the obtained gold nanoparticles, the gold sludge was first topped up with water after the centrifuge treatment so that a gold solid content of 25% by weight in the solution was achieved. The density of the ink was then at 1.3 g/cm³. The surface tension was lowered to 30 nNm by adding 0.05% by weight Byk 348.

The produced gold nanoparticle ink was printed on silicon wafers and on aluminum oxide substrates in 21 mm long meander test structures using a commercial ink jet device with a Dimatiax SE128 printhead. In this respect, the achieved line width amounted to 120 μm and the layer thickness in a simple print 1.5 μm. After a heat treatment above 300° C., a considerable sintering arose with a drop of the electrical resistance to 4.7 ohm. A specific electrical resistance of the printed gold track of 9.53 μohm-cm results from this which corresponds to a value of less than five times that of pure bulk gold at 2.21 μohm-cm.

EXAMPLE 3

10 g gold was completely dissolved in 13 ml of a hydrochloric acid/nitric acid mixture, as also in Example 3, for the production of gold nanoparticles with a mean particle size d₅₀ of approx. 20 nm. No metallic vessels or devices should be used there either due to the desired purity. This solution was filtered using a hard paper filter after cooling. 50 times the mass of deionized water with respect to the gold mass used was added for a precipitation of gold particles to be carried out and the total was mixed in a magnetic agitator. The surface tension ratios were in this respect influenced by the addition of Byk 180 and Byk 190 into the water as a surfactant. Both surfactants were added into the water with respectively 60% by weight of the gold used.

Subsequently, the initially produced acid mixture in which the gold was contained was added to the surfactant-water mixture and the whole was homogenized. A pH sensor was used for monitoring the pH.

This mixture was then heated to 60° C. and then 3-5 mol NaOH was added. After reaching a pH of 7, it then fell to a pH between 1.5 and 2. The pH reached is influenced by the surfactant fraction.

After a further metered addition of 1-mol NaOH, the pH was again stabilized at 7. If this pH was able to be held constant over a period of time between 5 and 10 minutes, the pH was increased to 10 by a further addition of NaOH and was held constant there.

The synthesis reaction came to an end; gold particles were precipitated.

The finer nanoparticles were then able to be separated by means of centrifuging, analog to Example 2.

The sediment can be separated by centrifuging and can subsequently be washed a multiple of times to completely remove Na and NaCl.

The non-used coarse fraction of the sediment can be recycled. Larger gold losses are thereby avoided.

EXAMPLE 4

In a first step, 10 g of a commercial platinum powder were completely dissolved in at least 16 ml of a mixture of 75% by weight HCl with 25% by weight HNO₃ at a temperature at the boiling point of nitric acid at approx. 86° C. Since other metals would likewise dissolve in the acid mixture and would effect a contamination of the platinum, no metallic devices should be used. After cooling, the solution was passed through a hard paper filter. 500 ml deionized water was mixed with surfactant (Disperbyk 180 and Disperbyk 190) using a magnetic stirrer in a glass beaker. The quantity of added surfactant corresponded to 100% by weight of the previously produced acid mixture. The acid solution was then added to the water-surfactant mixture and homogenized while stirring. The immersion of a pH electrode allowed the monitoring of the pH for the following synthesis. 25% hydrazine hydrate solution (70 . . . 80% of the mass of the platinum used) was then added to the mixture and the pH was titrated to a neutral pH by adding NaOH (3 . . . 5 mol). Foaming abruptly occurred in this process. As the reaction time increased, the pH fell from 7 back to a pH of 1 to 2. The pH was stabilized at pH 7 by further metering in of NaOH. No further pH change occurred after a further 5 to 10 minutes. The pH was thereupon increased to a pH of 9 by a further addition of NaOH and stabilized there. Platinum nanoparticles are precipitated in the solution during the increase in the pH. Coarser particle sizes were separated by using a centrifuge with 2000 r.p.m. (800 times the force of gravity g) for 10 min. The remaining liquid was centrifuged out at a speed of 4600 r.p.m. A fine sediment was obtained which can be washed and processed into a printable ink. The remaining platinum particle sludge was washed with water to reduce the sodium content in the solution.

EXAMPLE 5

The following procedure was carried out for the production of nanoparticles of platinum with a mean particle size d₅₀ of approx. 30 nm.

10 g platinum were completely dissolved in 12 ml of an acid mixture (75% by weight HCl and 25% by weight HNO₃). No metal vessels or devices should be used in this process either.

This solution was likewise filtered by means of a hard paper filter.

10 g of the surfactants Byk 180 and 190, which corresponds to the platinum mass used, were added to deionized water with a mass 50 times greater than that of the platinum used, that is a mass of water of 500 g. A mixture with the acid solution mixture was prepared from this and heated to 60° C.

After homogenization, 70 to 80% by weight of hydrazine hydrate, with respect to the platinum used, was added.

The pH was influenced by titrating 3-5 mol NaOH.

A pH of 10 was reached after a very short time and platinum particles were precipitated.

Nanoparticles in the desired particle size range were able to be separated from the sediment by centrifuging, as explained in Example 3.

Noble metal losses were also able to be reduced by recycling here.

EXAMPLE 6

In a first step, 13 g of pure silver powder (Heraeus 300-01) were dissolved in 14 ml of hot 65% nitric acid at 120° C. and 20 g AgNO₃ were acquired by a subsequent boiling down of this solution, with the former being crystallized out. Then these 20 g AgNO₃ were dissolved in 1600 ml water and 19.2 g of the dispersant Disperbyk 180 and 48 g Disperbyk 190 (both from the company Byk-Chemie) were added. The solution was heated to 35° C. and set to a pH of 9 with the aid of NH₃. Subsequently, 20 ml hydroxylamine 50% aqueous solution (type Merck, for synthesis) was added. A volume of at least 5 liters must be provided for the described reaction due to the strong gas development in the reaction. The reaction comes to a stop after approx. 30 s. In this time, the temperature in the solution increases to around 40° C. The reaction vessel is cooled to room temperature while stirring continuously and is subsequently centrifuged. A separation of coarse particles larger than 300 nm at an acceleration of 800 g for 10 min. is suitable for the described reaction conditions. Subsequently, centrifuging takes place for 2 hours at a maximum acceleration of the centrifuge, e.g. at 4600 g. A yield of 65-75% of the silver mass used in nanoparticles is thus possible. The particles which are too coarse for the application can furthermore be liberated from the adhering organic dispersant by burning out and can subsequently again be supplied to the process as pure silver.

The particles produced from the above-described synthesis are further processed to a silver particle ink. The centrifuge sediment is for this purpose, after centrifuging at 4600 g for 2 hours, charged with as little water as possible and dispersed in a ball mill for 0.5 hours. Additives such as 10% by weight PEG (polyethylene glycol) and 0.05% by weight Disperbyk (figures with respect to the mass of the sediment) can be added to improve the printability. To ensure the avoidance of later clogging of printing nozzles, the ink thus obtained was liberated from any coarse agglomerates with the aid of a 5 μm steel filter. The viscosity of the ink amounts to 18 mPas at a shear rate of 100/s and at 25° C. in the cylinder beaker system (TA Instruments, DA100). The solid content of the ink was calculated using a density measurement and amounted to 75% by weight at a density of 3.30 g/cm³. An FESEM shot of the Ag particles of this ink on polished Al is shown in FIG. 3.

This ink was deposited on silicon with the aid of a Dimatix SQ128 printhead. The layer thickness in a single print amounted to 3.2 μm, with a line width of 50 μm. In multiprint with 5 repeats, layer thicknesses of 16 μm are reached, with line widths of 60 μm. An FESEM shot of these printed layers is shown in FIG. 4. The printed structures were burnt in a tube furnace at a heating rate of 10 K/min up to a temperature of 1000° C. In this respect, the electrical resistance of the printed structure was determined using a 4-point measurement. The curve in dependence on the temperature is shown in FIG. 5. The printed layer already becomes conductive at temperature above 275° C. On sintering up to 500° C., a specific electrical resistance of the Ag conductive track of 0.05 Ωmm²/m was determined—this corresponds to an approximately threefold bulk Ag resistance 0.016 Ωmm²/m. This roughly corresponds to three times that of pure silver (0.016 0.05 Ωmm²/m) and thus represents a very good value for a printed layer. The Ag conductor track starts to melt above 949° C., with the electrical conductor track resistance increasing greatly.

Unlike Example 1, a “coarser” ink with an extremely high solid content can be produced using the silver particles obtained in accordance with Example 6. This achievable solid fraction in printable inks of up to 75% by weight greatly surpasses that of conventional printable inks with silver nanoparticles. A solid fraction of a maximum of 40% by weight is known from them.

EXAMPLE 7

In a first step, 13 g pure silver powder (Heraeus 300-01) were dissolved in 14 ml of hot 65% nitric acid at 120° C. The solution is then diluted with 1600 ml water and 19.2 g dispersant Disperbyk 180 and 4.8 g Disperbyk 190 (both from Byk-Chemie) are added. The solution is heated to 35° C. and set to a pH of 9 by the controlled addition of NH₃. Subsequently, 20 ml hydroxylamine 50% aqueous solution (type Merck, for synthesis) is added. A volume of at least 5 liters must be provided for the described reaction due to the strong gas development in the reaction. The reaction comes to a stop after approx. 30 s. In this time, the temperature in the solution increases to around 40° C. The reaction vessel is cooled to room temperature while being stirred continuously and is subsequently centrifuged. A separation of coarse particles larger than 300 nm at an acceleration of 800 g over a period of 10 min. is suitable for the described reaction conditions. Subsequently, centrifuging takes place for 2 hours at a maximum acceleration of the centrifuge, e.g. at 4600 g. A yield of 65-75% of the silver mass used in nanoparticles is thus possible. The particles which are too coarse for the application can furthermore be liberated from the adhering organic dispersant by burning out and can subsequently again be supplied to the process as pure silver.

The particles produced from the above-described synthesis are further processed to a silver particle ink. The centrifuge sediment is for this purpose, after centrifuging at 4600 g for 2 hours, charged with as little water as possible and dispersed in a ball mill for 0.5 hours. Additives such as 10% by weight PEG (polyethylene glycol) and 0.05% by weight Disperbyk (figures with respect to the mass of the sediment) can be added to improve the printability. To ensure the avoidance of later clogging of printing nozzles, the ink thus obtained was liberated from any coarse agglomerates with the aid of a 5 μm steel filter. The viscosity of the ink amounts to 20 mPas at a shear rate of 100/s and at 25° C. in the cylinder beaker system (TA Instruments, DA100). The solid content of the ink is calculated using a density measurement and amounted to 74% by weight at a density of 3.29 g/cm³. 

1. A method for producing nanoparticles from a noble metal, which is selected from silver, gold, and platinum, having an adjustable particle size, the method comprising: dissolving a chemical compound of the respective noble metal in an aqueous solution or dissolving the noble metal in an acid mixture; adding an aqueous or alcoholic solution containing at least one surfactant to the respective solution and additionally adding a reductant with silver and platinum; wherein the particle size of the nanoparticles is influenced by influencing the parameters: concentration of the chemical compound or of the noble metal, temperature, pH, and the fraction of surfactant; and centrifuging out the noble metal particles precipitated from the respective solution.
 2. The method in accordance with claim 1, wherein the particle size is reduced with a smaller concentration of the chemical compound used or of the noble metal, and/or at an elevated temperature and/or with an increased fraction of surfactant.
 3. The method in accordance with claim 1, wherein the particle size is reduced with an increased pH.
 4. The method in accordance with claim 1, wherein, for the production of nanoparticles of gold or platinum, the respective metal is dissolved in a mixture of hydrochloric acid and nitric acid.
 5. The method in accordance with claim 1, wherein, for the production of nanoparticles, the silver is dissolved in hot nitric acid, preferably at a temperature in the range from 100° C. to 150° C.
 6. The method in accordance with claim 1, wherein the at least one surfactant is selected from alkoxylates, alkylolamides, esters, amine oxides, alkyl polyglucosides, alkylphenols, arylalkyl phenols, water-soluble homopolymers, water-soluble statistical copolymers, water-soluble block copolymers, water soluble graft polymers, polyvinyl alcohols, copolymers of polyvinyl alcohols and polyvinyl acetates, polyvinylpyrrolidones, cellulose, starch, gelatins, gelatin derivatives, amino acid polymers, polylysine, polyasparagine acid, polyacrylates, polyethylene sulfonates, polystyrene sulfonates, polymethacrylates, condensation products of aromatic sulfonic acids with formaldehyde, napththalenesulfonates, lignosulfonates, copolymerizates of acrylic monomers, polyethyleneimines, polyvinylamines, polyallylamines, poly(2-vinylpyridines), block copolyethers, block copolyethers with polystyrene blocks, sodium dodecylbenzenesulfonate, and polydiallyl dimethyl ammonium chloride.
 7. The method in accordance with claim 1, wherein hydroxylamine is used for the production of nanoparticles from silver and hydrazine hydrate or sodium borohydride is used as a reductant for the production of nanoparticles from platinum.
 8. The method in accordance with claim 1, wherein the pH in the solution is set with added NaOH, added NH₃ and/or added piperidine.
 9. The method of claim 1, comprising using the nanoparticles for the production of electrically conductive and/or printable ink, and/or electrically conductive and optically reflective layers. 